1. Field of Invention
This invention pertains to apparatus and method for reading helically recorded magnetic tape, and particularly to apparatus and method for recovery from errors that occur when reading helically recorded magnetic tape having distorted tracks.
2. Related Art and Other Considerations
In accordance with helical scan recording technology, magnetically reproducible signals are recorded on magnetic storage media in the form of parallel tracks or stripes. It is well known that operational problems may cause one or more tracks of information, helically recorded on magnetic tape storage media, to appear upon playback or reading as a distorted track.
One type of distorted track is a curved track. In this respect, problems such as those associated with the handling or guiding of a magnetic tape as it is being read may cause a track to appear as a curved track.
Various prior art schemes have been developed to compensate for the reading of curved tracks. These schemes primarily enable a track-reading head or transducer to follow the curvature of the track. Typically this is done by mounting the track-reading head upon an element (such as a bi-morph leaf) that can be deflected to permit the head to follow the curved track.
The prior art schemes described above generally require that the track be formatted upon recording to include not only the stored data information, but also a special tracking or servo signal which must be continuously or periodically recorded along the length of the track. One examples of such a scheme is illustrated in U.S. Pat. No. 5,068,757 to Hughes et al. (incorporated herein by reference), which describes a servo technique utilized by an EXB-8200 helical tape drive manufactured by Exabyte Corporation. Other examples include U.S. Pat. Nos. 4,486,796 to Sakamoto; 4,420,778 to Sakamoto; 4,404,605 to Sakamoto; 4,172,265 to Sakamoto et al.; 4,099,211 to Hathaway; and, 4,106,065 to Ravizza.
Not all helical scan recording systems provide a continuous or periodic tracking or servo signal along the length of a track. For example, U.S. Pat. No. 4,843,495 to Georgis et al. (entitled CYCLICAL SERVO ZONE TRACKING METHOD AND APPARATUS FOR HELICAL SCAN RECORDING DEVICES), commonly assigned herewith and incorporated by reference herein, describes a servo technique implemented on an EXB-8200 helical tape drive manufactured by Exabyte Corporation. That servo technique involves the recording of a servo signal essentially only at the beginning of each track. Accordingly, the prior art schemes for attempting to read curved tracks are not suitable for this particular servo technique, or any technique (such as the Sony NT format) which does not employ servo signals at all.
Downtrack from the servo signal area, both the prior art EXB-8200 helical tape drive and the prior art EXB-8500 helical tape drive record informational data in a blocked format, with eight data blocks being recorded per track or stripe. As explained in U.S. patent application Ser. No. 07/069,132 filed Jul. 2, 1987, now abandoned, entitled METHOD AND APPARATUS FOR DATA BUFFER MANAGEMENT, commonly assigned herewith and incorporated herein by reference, the magnetic tape is formatted so that every data block has a header portion that includes block-identifying information. Upon reading of a block, a micro-controller stores the data contents of the block in a data buffer and the block-identifying information into an allocation table. Using the block-identifying information stored in the allocation table, the micro-controller ascertains the order for utilizing the corresponding data blocks stored in the data buffer.
If a search of the allocation table of the EXB-8200 indicates that a block is missing, as might occur during a curved track read condition, the micro-controller would request re-reading of the tape in hopes that the missing block would be detected and read upon re-read. However, during re-read the read head would follow essentially the same path as during the initial read, with the result that a block on a curved track that evaded discovery during an initial read attempt could also be evasive during subsequent read attempts.
U.S. Pat. No. 5,050,018 to Georgis et al. (incorporated herein by reference) teaches a re-reading of tape with distorted tracks in a manner whereby the read head is positionally offset from its original azimuthal paths so as to follow modified azimuthal paths at nominal tape speed. The tape is repeatedly re-wound, if necessary, with a different modified azimuthal path being followed after each rewind, until all blocks have been read. However, under some circumstances, repeated re-winding of the tape medium can exacerbate track distortion and can be deleterious to the tape medium.
Sony Corporation has introduced an "NT" format wherein read heads travel, at greater than a nominal recording speed, over tracks without necessarily passing over the track centerlines, and thus not necessarily reading the blocks in the order in which they were written. By using block identification information embedded in each block of the track, an apparatus using the NT format places the read blocks in an intelligible order for utilization.
In addition to curved tracks, other types of distortions to the ideal track geometry may be present when a tape is read. These distortions may be the result of the data write operation, the read operation, or both. Some examples of these distortions are listed below:
(a) Track pitch distortions caused during the recording process by fluctuating tape speed or abnormal splice operations.
(b) Track angle variations caused by tape guide misalignment or by the use of tape having worn or damaged edges. This could effect both the data write and read operations.
(c) Interchange between two data recorders having incompatible tape guide adjustments. This could result in combinations of track angle and track curvature problems during the data read process.
(d) Other types of distortion could result from contaminants becoming deposited on the tape after the data was recorded. The presence of the contaminant could alter the way in which the read head follows the recorded track.
Accordingly, it is an object of the present invention to provide method and apparatus for reading helically recorded tracks, even when the tracks are distorted.
An advantage of the present invention is the provision of method and apparatus that facilitates the reading of distorted tracks, such as curved tracks, when tracking or servo information is not provided continuously or periodically along the tracks.